The head mounted projection display (HMPD), as an alternative to the conventional eyepiece-based head mounted display (HMD), has attracted much interest in recent years, because it offers the ability to design a wide field of view (FOV), low distortion and ergonomically compact optical see-through head mounted display (OST-HMD). Like most OST-HMDs, however, one of the limiting factors for the HMPD technology is its low image brightness and contrast, which limits the feasibility to apply such information to outdoor or well-lit indoor environments such as operating rooms. Due to the multiple beamsplitting through a beamsplitter and low retroreflectivity of typical retroreflective materials, the overall efficiency of a HMPD is around 4%. For instance, with a miniature backlit active matrix liquid crystal display (AMLCD) as the image source, the luminance of the observed image is estimated to be 4 cd/m2, while the average luminance of a well-lit indoor environment is over 100 cd/m2. As a result, the low-brightness image of HMPDs will appear washed out in such well-lit environments. In fact, most optical see-through HMDs, including HMPD, are typically operated under a dimmed lighting condition.
To address this problem, a polarized head-mounted projection display (p-HMPD) was proposed (H. Hua and C. Gao, “A polarized head-mounted projective displays,” Proceedings of 2005 IEEE and ACM International Symposium on Mixed and Augmented Reality, pp. 32-35, October 2005) and a prototype based on a pair of transmissive AMLCDs was designed recently (H. Hua, C. Gao “Design of a bright polarized head-mounted projection display” Applied Optics, Vol. 46, Issue 14, pp. 2600-2610, May 2007). A pair of 1.3″ color AMLCDs was used as the image sources which have a resolution of (640*3)*480 pixels. 1.4″ Alphalight™ RGB LED panels (Teledyne Inc., Los Angeles, Calif.) were used as the backlighting sources. By carefully manipulating the polarization states of the light propagating through the system, a p-HMPD can potentially be three times brighter than a traditional non-polarized HMPD design using the same microdisplay technologies. A schematic design of a monocular p-HMPD configuration is illustrated in FIG. 1.
The image on the LCD display is projected through the projection lens, forming a real intermediate image. The light from the LCD is manipulated to be S-polarized so that its polarization direction is matched with the high-reflection axis of the polarized beamsplitter (PBS). After the projected light is reflected by the PBS, it is retroreflected back to the same PBS by a retroreflective screen. The depolarization effect by the retroreflective screen is less than 10% within ±20 degrees and is less than 20% up to ±30 degrees. As a result, the retroreflected light remains dominantly the same polarization as its incidence light. In order to achieve high transmission through the PBS after the light is retroreflected back, a quarter-wave retarder is placed between the PBS and the retroreflective screen. By passing through the quarter wave retarder twice, the incident S-polarized light is converted to P-polarization and transmits through the PBS with high efficiency. Thus the projected image from the microdisplay can be then observed at the exit pupil of the system where the eye is placed.
However, since a transmissive LCD microdisplay has a low transmission efficiency of around 5%, the overall performance of the first p-HMPD prototype is still unsatisfactory in a well-lit environment. Furthermore, owing to its inherent low pixel fill factor, a transmissive AMLCD microdisplay typically has a relatively low resolution. Accordingly, it would be an advance in the field of head-mounted projection displays to provide a head-mounted projection display which has higher luminance while maintaining high contrast.